This invention relates to a semisubmersible column stabilized vessel and particularly to a twin hull column stabilized semisubmersible drilling vessel designed for low cost optimum construction and use.
A significant number of drilling operations have been conducted in offshore areas where a substantial body of water overlies the oil and/or gas field. For example, over 10,000 wells have been drilled offshore and oil and gas is presently piped to on-shore terminals from as far as 250 miles out to sea and in water depths up to 500 feet. Increasing worldwide demand for hydrocarbons has pushed the frontiers of offshore drilling activities further and further into deeper waters, more hostile seas, and drilling sites even greater distances from the shore. This has resulted in a trend over the past few years toward the construction of larger and more self-sustaining drilling vessels. Concommitantly, the offshore industry has incurred the high costs and long delivery times associated with the construction of such larger vessels. For example, construction costs have been growing in recent years at an average rate of 25% per year and the present costs of such large semisubmersible type drilling vessels typically are upwards of $40-$50 million and higher. The foregoing factors of increased cost, long delivery time, and increasing demand for immediate discovery and exploitation of oil and gas wells offshore and other factors have demonstrated the need for a vessel combining optimum performance, i.e. a vessel capable for performing drilling operations in most of the geographic areas in which hydrocarbons may be found offshore, with minimum construction costs and delivery time.
The present semisubmersible column stabilized drilling vessel constitutes an optimum vessel design for low cost construction with capability for conducting drilling operations in most geographic areas in the world. By imposing a few geographic constraints on the vessel's operation, the present vessel can be built with mass production techniques in minimum facility shipyards at low costs, and particularly at costs and construction time substantially reduced in comparison with the costs and construction time of present day large semisubmersible type vessels. More particularly, by excluding vessel operations in particularly harsh offshore areas of the world, a low cost optimum vessel is provided with efficient vessel configuration and layout to achieve optimum size for initial investment, minimum steel weight, effective motion reduction and stability, and motion characteristics equivalent to or better than much larger semisubmersible vessels.
Particularly, the present vessel design includes a pair of elongated lower hulls in transverse spaced side-by-side relation and supporting a platform in spaced relation above the hulls by a pair of columns adjacent the opposite ends of the hulls and longitudinally spaced trusses interconnecting the hulls, columns and platform. The vessel may be ballasted to change its draft from a low draft in-transit floating condition with the hulls having freeboard to a high draft floating and drilling condition with the hulls and lower portions of the columns and trusses submerged below the mean waterline such that the latter lies along an intermediate portion of the column height. In the high draft condition, the columns provide righting moments about pitch and roll axes. Also the number, cross-sectional area, and configuration of the columns together with the weight distribution of the vessel and the geometry of the submerged hulls and portions of the columns are such that vessel motion is minimized in the high draft condition. The platform carries a centrally located drilling mast and ancillary drilling equipment and drilling operations are conducted with the drilling line extending downwardly from the platform between the hulls.
In optimizing the vessel design in accordance with the above, the vessel obtains certain characteristic features. For example, to provide adequate stability in the high draft condition, minimize vessel motion, minimize use of steel, and provide low cost construction and other features, the hydrostatic properties of the vessel should stand in relation to its geometric properties such that the ratio of the righting moment about the transverse pitch axis of the vessel to the righting moment about the longitudinal roll axis of the vessel when the vessel is in high draft semisubmerged operating condition lies within a range of 1.0 to 1.3 while the geometric ratio of the length of the vessel to its width should lie within a range of 1.2 to 1.5.
Further, the trusses, and the platform and drilling mast are aligned, interconnected, and interrelated to achieve high longitudinal and transverse structural strength and integrity with reduced steel weight. For example, high structural strength in the transverse direction is afforded by the alignment at each of the opposite ends of the forward and aft pairs of columns of a transverse bulkhead plate in each hull, the end shell plating of each columns, a continuous transverse bulkhead in the platform extending the width of the platform and the trusses which interconnect the hulls, platform and columns. Longitudinal structural strength is provided by the alignment of longitudinally extending shell plates in the platform with shell plates along the inner and outer sides of the columns and with longitudinally extending arches in each hull. Further, support columns directly interconnect the hulls and the platform drill floor and substructure. The majority of the structural support for the substructure including drilling mast and ancillary equipment is thus transmitted directly to the hulls. The foregoing structural arrangement minimizes the steel weight necessary to construct the vessel.
Still further, the present vessel design has a natural period in heave in a range of 16-18 seconds which is well outside the range of periods of normally occurring waves. More importantly, however, the present vessel obtains a heave response in terms of the ratio of heave amplitude to wave amplitude no greater than 0.60 for wave periods less than 14 seconds and within a range of 0.35 to 0.60 in the range of waves of 10-14 seconds. This heave response is at least as good as and in many instances better than the heave response of such larger and costlier semisubmersible vessels and this is achieved simultaneously with the achievement of the foregoing and other features of the present invention described herein whereby a low cost optimum semisubmersible drilling vessel is provided.
Accordingly, it is a primary object of the present invention to provide a novel and improved column stabilized semisubmersible drilling vessel of optimum size and performance.
It is another object of the present invention to provide a novel and improved column stabilized semisubmersible drilling vessel having high transverse and longitudinal structural strength with minimum steel weight.
It is still another object of the present invention to provide a novel and improved column stabilized semisubmersible drilling vessel including optimization of its geometric and hydrodynamic characterisitcs.
It is a further object of the present invention to provide a novel and improved column stabilized semisubmersible drilling vessel which, for a vessel of its size and configuration, has motion characteristics equivalent to or better than larger semisubmersible type vessels.